, research means to observe the phenomena again and again from different dimensions.

, research means to observe the phenomena again and again from different dimensions. For example there are many theories of learning due to the observation from different dimensions. The research is a process of which a person observes the phenomena again and again and collects the data and on the basis of data he draws some conclusions.

1. CELL STRUCTURE AND ORGANIZATION

2. The Cell: The cell structural and functional unit of life. All living organisms are made up of one ( unicellular) or many ( multicellular ) cells. There are two basic types of cells: prokaryotic and eukaryotic cells. In prokaryotic cells the nucleus is primitive type i.e. it is not surrounded by the nuclear envelope. In eukaryotic cells nucleus is surrounded by nuclear envelope. Eukaryotic cells are divided into two types: Plant cells and animal cells. Plant cells have cell wall while animal cells lack cell wall. Fungi and bacteria have cell wall.

3. Structure of prokaryotic cell

4. Animal cell Plant cell Structure of eukaryotic cell

5. Structure of virus

6. The cell was discovered by Robert Hook in 1665 from the cork of wood. Later the cell theory was given Scheilden and Schwann postulated cell theory in 1839. According to this theory 1. ‘ cell is basic structure and function life’. 2. all cells are derived from pre existing cells. 3. an organism can be made up single cell ( unicellular) or many cells ( multicellular). Modern cell theory: All known living organisms are made up of one or more cells. The cell is structural and functional unit of life. All cells arise from pre- existing cells by cell division. Energy flow occurs within the cell.

7. 5. Cells contain heredity information (DNA) which passes from one cell to other. 6. All cells have basically the same chemical composition. The earliest cell was arose probably around 3.5 billion years ago from a rich mixture of organic compounds. The first cells were chemoheterotrophs. Initially only prokaryotic cells were present. Later around 1.5 billion years ago eukaryotic cells came into existence.

8. Changes occurred during formation of eukaryotic cell from prokaryotic cells: The cells acquire more DNA, mechanisms evolved for folding of DNA and formation of complex proteins. More complex cell division mechanism. Cells became larger and intracellular membrane developed. Development of more cell organelles and association of them for various complex mechanisms. Initially eukaryotic cells were chemotrophic. Later during course of evolution some cells acquire phototrophic unicellular organisms and established them in the cell. These started acting as chloroplast. And the eukaryotic cells became phototrophic.

9. Cell wall : It present in prokaryotes like bacteria & made up of peptidoglycans. In eukaryotes fungi, plants show presence of cell wall. Plant cells are surrounded by cell wall which gives mechanical strength and protection against various biotic and abiotic factors. Plant cell wall plays an important role in growth, development, maintenance and reproduction of plants. Cell wall acts as glue and keeps the cells together without sliding them on one another. It helps in maintenance of cell shape by keeping high turgor pressure.

10. Plant morphogenesis depend upon the control of cell wall because the expansive growth of plant cells is limited by the ability cell wall to expand. The bulk flow of water in xylem requires tough wall to resist collapse by the negative pressure in the xylem. The cell wall acts as diffusion barrier that limits the size of macromolecules that can reach the plasma membrane from outside. Cell wall is major structural barrier for pathogens.

11. Structure of cell wall: Three layers of cell wall are present in plant cells viz. primary cell wall, secondary cell wall and tertiary cell wall. Cell wall composition varies with types of plant cells as well as age of plants. Parenchymatous cells have thin cell wall made up of mainly cellulose and hemicellulose while in cells like epidermal cells, phloem fibers, xylem tracheids, collenchyma cells the cell wall is multilayered and made of lignin, cutin, suberin, silica, wax or other structural proteins. The thickness of cell wall also varies with respect the position. Eg. The plasmodesmata region lacks cell wall and it is impregnated with cutin & wax.

12. Cell wall is divided into two types: primary cell wall and secondary cell wall. Primary cell wall : It is formed in growing cells and show almost similar structure in all cells. Primary cell wall is made up of cellulose microfibrils embedded in a highly hydrated matrix. It consists of two major polysaccharides viz. cellulose & hemicellulose. The matrix is made up of pectins and some of structural proteins.

13. The matrix polysaccharides vary with respect to polymer structure and variety of polymers from species to species. Primary cell wall is composed of approximately 25% cellulose, 25% hemicellulose, 35% pectins and 1% to 8% of structural proteins. Cellulose : It is a polysaccharide of glucose residues linked together by 1-4 linkage of β- D glucose molecules ( i.e. Glucan). These polysaccharides are normally named after the principle sugars the contains eg. Polymer of glucose = glucan, polymer xylose= xylan , polymer of galactose= galactan . Sometimes two or more sugars form the backbone of a polymer. Eg. Xyloglucan is polymer of xylose and glucose.

14. Cellulose microfibrils are stiff and tough in nature which contribute strength and structural bias to the cell. The glucans are closely aligned and bonded to each other to form a strong crystalline structure. Cellulose microfibrils are of indeterminate length and considerably vary in width and degree of order, depending upon the source.eg. Land plants have cellulose microfibril of width about 5 to12nm while in case of algae it is up to 30nm. Each microfibril may contain 20 to 40 individual chains of polymer. Cellulose microfibril

15. Structure of cellulose & Biosynthesis: Cellulose has a substructure of highly crystalline domains linked together. These are highly crystalline structure linked together with amorphous regions. The adjacent glucan molecules are highly ordered and bonded to each other by hydrophobic interactions and hydrogen bonds.

16. One polymer is made up of about 25000 to 40000 glucose residues. Cellulose shows high tensile strength as steel. It is insoluble in water, chemically stable and relatively immune to chemical and enzymatic attacks. Biosynthesis of Cellulose : Biosynthesis of cellulose starts in Golgi complexes and the basic sugar is uridine diphosphate D –glucose ( UDP- glucose). Sucrose also acts as the starting molecule for glucose biosynthesis during cellulose formation. It is converted in to glucan polymers cellulose synthase present in cytoplasmic site of plasma membrane.

17. After formation of glucan molecules the sterol molecules are clipped off by enzyme endoglucanase . Then the growing glucan molecule is transferred on outer side of plasma membrane and newly synthesized polymer starts binding with the pre existing polymer. These crystalline polymers move in amorphous region (matrix of cell wall). This amorphous matrix is made up of hemicellulose, pectins and structural proteins. The components of matrix are synthesized by Golgi complexes and are transferred to cell wall by exocytosis through small vesicles.

18. Structure of Hemicellulose : It is a heterogenous group of polysaccharides that is bound to cell wall tightly. It has a back bone of cellulose polymer to which short side chains are of xylose, arabinose, galactose are attached. These side chains interfere the alignment and bonding between glucan polymers and hence keep the structure in amorphous form. These structure not form crystalline microfibril as the side chains may contain 50 - 500 xylose residues.

19. Pectins: Pectins are heterogenous polysaccharides containing acidic sugars such as galacturonic acid and neutral sugars such as rhamnose, galactose & arabinose. Theses are most water soluble cell wall polysaccharides & can be extracted by using hot water or calcium chelators.

20. Some pectins have simple structure eg. Homogalacturonan. It is a polymer of α - D glucuronic acid having 1-4 linkage. Pectins also contain some complex polysaccharides such as rhamnogalacturonan I (RG I) which long backbone and many side chains making highly branched structure. Another highly branched complex polysaccharide is rhamnogalacturonan II ( RG II). RG II units are linked together by borate diesters and are important for cell wall structure. Pectins are secreted in cell wall the ester groups are removed by enzyme pectin esterase. This leads to unmasking of carboxyl groups present polysaccharides & increases the ability of the pectin to form a rigid gel.

21. Cell wall structural proteins: Along with major polysaccharides cell wall contains certain proteins. These proteins are classified on the basis major amino acid composition. Eg. Hyroxyproline –rich glycoproteins (HRGP), Glycine rich glycoproteins (GRP), Proline rich glycoproteins (PRP). The concentration of structural protein and their composition varies with respect to cell type, maturity of cell & nature of stimulation.eg. HRGP are abundant in cambium, phloem parenchyma. GRPs & PRPs are localized in xylem cells. Along with theses cell wall contains arabinogalactan proteins (AGPs). These are highly water soluble proteins made up of galactose and arabinose.

22. AGPs play role in cell adhesion as well as cell signaling during cell differentiation. AGPs are also implicated in growth, nutrition, guidance of pollen tube through stylar tube. AGPs are involved in various developmental processes.

23. Cell wall assembly: Primary cell originates de novo during the final stage of cell division, when cell plate separates two daughter cells & solidifies stable cell wall. The cell plate forms when Golgi vesicles and ER cisternae aggregate at midzone area of dividing cell. The membrane vesicles fuse with each other & with lateral plasma membrane. The vesicles contain precursors from which new middle lamella and primary cell wall is assembled. Two ways of primary cell wall assemble: 1. Self – assembly : This method does not involve enzymatic action for assembly of cell wall. Cell wall polysaccharides aggregate spontaneously into organized structures.

24. 2. Enzyme- mediated assembly : The cell wall components are assembled with help of various enzymes. The enzyme xyloglucan endotransglycosylase (XET) is prime enzyme in cell wall assembly. Other wall enzymes are glycosidase, pectinrethyl esterase, oxidases etc. Pectin methyl esterase help in ‘debranching’ of pectins & unblock carboxyl groups of pectins. This unblocking helps in binding of acidic groups to form calcium bridges in the gel network.

25. Secondary cell wall formation: It starts when cell expansion is completed and cell requires mechanical strength. Secondary cell wall is multilayered and differs in composition from cell to cell as well as from plant to plant. Eg. Secondary cell wall of wood contains more xylan than xyloglucans. The orientation of cellulose microfibrils more neatly and tightly aligned than primary cell wall. Secondary cell wall mostly contain lignin. Lignin is phenolic polymer complex linked with aromatic alcohol in irregular manner. The precursor for lignin phenylalanine. It is secreted in cell wall and oxidized by enzyme peroxidases .

26. The lignin forms in the cell wall, displaces water from it. It makes a hydrophobic network that bond with cellulose & prevents cell wall enlargement. Lignin also provides significant mechanical strength and reduces the susceptibility of walls to attack by pathogen.